Asymmetrically Branched Polymers
In recent years, a new class of polymers called dendritic polymers, including both Starburst dendrimers (or Dense Star polymers) and Combburst dendrigrafts (or hyper comb-branched polymers), have been developed and extensively studied in industrial and academic laboratories (Dendritic Molecules, edited by G R Newkome et al., VCH, Weinheim, 1996, and Dendrimers and Other Dendritic Polymers, edited by J M J Frechet and D A Tomalia, John Wiley & Sons, Ltd., 2001). These polymers often exhibit: (a) a well-defined core molecule, (b) at least two concentric dendritic layers (generations) with symmetrical (equal) branch junctures, and (c) exterior surface groups, as described in Tomalia's U.S. Pat. Nos. 4,435,548; 4,507,466; 4,568,737; 4,587,329; 5,338,532; 5,527,524; and 5,714,166, and the references therein.
These symmetrically branched dendrimers are also distinctively different from the previously prepared asymmetrically branched dendrimers (Denkewalter's U.S. Pat. Nos. 4,289,872; 4,360,646; and 4,410,688). The latter possess asymmetrical (unequal) branch junctures.
Both types of dendrimers can be produced by repetitive protecting and deprotecting procedures through either a divergent or a convergent synthetic approach. Since both symmetric and asymmetric dendrimers utilize small molecules as molecular building blocks for the cores and the branches, the molecular weights of these dendrimers are often precisely defined. In the case of lower generations, a single molecular weight dendrimer is often obtained.
Similar to dendrimers, Combburst dendrigrafts are also constructed with a core molecule and concentric layers with symmetrical branches through a stepwise synthetic method. In contrast to dendrimers, Combburst dendrigrafts or polymers are generated with monodisperse linear polymeric building blocks (Tomalia's U.S. Pat. No. 5,773,527 and Yin's U.S. Pat. Nos. 5,631,329 and 5,919,442). Moreover, the branch pattern is also very different from that of dendrimers. For example, Combburst dendrigrafts form branch junctures along the polymeric backbones (chain branches), while Starburst dendrimers often branch at the termini (terminal branches). Due to the utilization of living polymerization techniques, the molecular weight distributions (Mw/Mn) of these polymeric building blocks (core and branches) are often very narrow. As a result, Combburst dendrigrafts, produced through a graft-upon-graft process, are rather well defined with molecular weight distributions (Mw/Mn) often less than 1.2.
Although possessing well controlled molecular architecture, such as well defined size, shape, and surface functional groups, both dendrimers and dendrigrafts can only be produced through a large number of reiteration steps, making them only useful for esoteric academic studies rather than large scale commercial applications.
Dendrimers and dendrigrafts have been shown to possess unique carrier properties for bioactive molecules, as described in Tomalia's U.S. Pat. Nos. 5,338,532; 5,527,524; and 5,714,166 for Dense Star Polymers, and Yin's U.S. Pat. No. 5,919,442 for Hyper Comb-Branched Polymers. These unique properties (i.e., surface functional groups and interior void spaces) have been primarily attributed to the well-controlled, symmetrical dendritic architecture with predictable branching patterns (either symmetrical termini or polymeric chain branching) and molecular weights.
According to these teachings, random and regular, asymmetrically branched polymers (ran-ABP and reg-ABP) have long been considered as poor carrier materials. For example, a ran-ABP possesses: a) no core, b) functional groups both at the exterior and in the interior, c) variable branch lengths and patterns (i.e., termini and chain branches), and d) unevenly distributed interior void spaces. Although a reg-ABP possesses a core, the functional groups are both at the exterior and in the interior. Therefore, both ran-ABP and reg-ABP are generally considered to be unsuitable for carrying bioactive molecules.
The preparation of reg-ABP made of polylysine has been described, as illustrated in U.S. Pat. Nos. 4,289,872; 4,360,646; and 4,410,688.
The synthesis and mechanisms of ran-ABPs, such as made of polyethyleneimine (PEI), have been extensively studied (see G D Jones et al., J. Org. Chem. 9, 125 (1944), G D Jones et al., J. Org. Chem. 30, 1994 (1965), and C R Dick et al., J. Macromol. Sci. Chem., A4 (6), 1301-1314, (1970)).
The synthesis and characterization of random asymmetrically branched polymers, such as made of polyoxazoline, i.e., poly(2-methyloxazoline) and/or poly(2-ethyloxazoline), have been extensively studied by Litt (J. Macromol. Sci. Chem. A9(5), pp. 703-727 (1975)) and Warakomski (J. Polym. Sci. Polym. Chem. 28, 3551 (1990)).
Most of the prior art involved the utilization of polyethyleneimine polymers as coating materials to alter the characteristics of solid surfaces (i.e. changing charges, charge densities, and hydrophobicity). The coating aspects of polyethyleneimine polymers have been described in J Ness's U.S. Pat. No. 6,150,103 and K Moynihan's U.S. Pat. No. 6,365,349. Polyethyleneimines have also been tested as to carrying DNA molecules for gene transfection studies. However, the polymer was found to be cytotoxic.
Randomly branched poly(2-ethyloxazoline) has also been utilized to physically encapsulate protein molecules (U.S. Pat. No. 6,716,450). However, such an approach was not designed for the direct, covalent linking of ABP with bioactive materials for bioassays and drug delivery applications.
So far, none of the existing prior art has utilized modified ran-ABP and reg-ABP to carry bioactive materials for drug delivery and target recognition purposes, particularly for assay and microarray related applications, wherein transporting, anchoring, and orienting biologically active materials from a solution onto a solid surface all occur at the same time.
Assays and Microarrays
Since the completion of the human genome project, more and more researchers have realized that the elucidation of biological pathways and mechanisms at the protein level is actually far more important than at the genetic level. This is because the former is more closely related to different diseases and disease stages. With this strong demand push, a new forum called proteomics has recently become a major research focus for both industrial and academic researchers.
Currently, three major research tools have been employed in the proteomics research arena, primarily for the discovery, high throughput screening, and validation of new protein targets and drug leads. These tools include two dimensional (2-D) gel electrophoresis, mass spectrometry, and more recently, protein microarrays. In contrast to the lengthy 2-D gel procedures and tedious sample preparation (primarily separations) involved in mass spectrometry analysis, protein microarrays provide a fast, easy, and low-cost method to screen large amounts of proteins, as well as their functions. Therefore, microarrays are highly desired by proteomics researchers.
However, the protein-based microarray technology is far less developed than gene microarrays. The construction of a protein/antibody chip presents daunting challenges not encountered in the development of classical immunoassays or of DNA chips. In general, proteins are more sensitive to their environment than nucleic acids. The hydrophobicity of many membrane, glass, and plastic surfaces can cause protein denaturation, rendering the capture molecules inactive and resulting in lower sensitivity and higher noise-to-signal ratios. In other words, to construct a protein microarray, one must be able to overcome at least three major problems, protein denaturation, immobilization, and orientation.
For example, a protein molecule often folds into a three-dimensional structure in solution for and to maintain biological activity. On interaction with different solid surfaces, for example, during immobilization of proteins onto membranes, glass slides, or micro/nanoparticles, the three-dimensional structure of the protein molecule often collapses, thus losing biological activity. In addition, proteins often do not have the ability to adhere onto different surfaces.
To immobilize the protein molecule on a surface, a direct covalent linking reaction or an electrostatic interaction (physical adsorption) often has to be employed. Heterogeneous chemical reactions often are incomplete yielding undesired side products (i.e. incomplete modification of surfaces), and in some cases, also partially denatured proteins during different reaction stages.
The electrostatic interaction relies heavily on the isoelectric points of the proteins, as well as the pH of the buffer solutions.
Both approaches tend to give irreproducible results due to the complexity involved in these procedures. The lot-to-lot reproducibility is, therefore, very poor. As a result, there is a great interest in modifying solid substrates, but not the protein molecule itself. A variety of polymers, including polyethyleneimine polymers, have been utilized as coating materials to alter the characteristics of solid surfaces for the construction of protein arrays, as described by P Wagner et al. in U.S. Pat. No. 6,406,921.
So far, none of the prior art utilizes modified random and regular asymmetrically branched polymers as carriers for bioactive materials, particularly for the construction of assays and microarrays.